Applications such as AC/DC power supply with alternating current input or LED driver or the like have a power supply capacitor. The charge stored in the power supply capacitor is used for controlling an internal circuit of a chip and driving a power switch. When a field effect transistor is driven to be turned on, it is required to quickly drive its gate voltage to an appropriate voltage (typically 5V-15V) to achieve a small on-resistance between the drain and source.
FIG. 1 shows an examplary circuit relating to a power switch drive circuit in the prior art. The power switch drive circuit includes a rectifier bridge (including a diode D1, a diode D2, a diode D3, and a diode D4), a bus capacitor Cbus, an inductor Lm. a freewheel diode Dr, an output capacitor Cout, a load Rload, and a drive chip; the drive chip further includes a power switch M1, a junction field effect transistor JFET, a JFET control circuit, and a drive circuit; of which the source of the junction field effect transistor JFET is connected with a power supply capacitor Cvcc. The power supply capacitor Cvcc supplies power to the driver circuit and drives the gate of the power switch M1. A source of the power switch M1 is connected with a sampling resistor Rcs.
Since the drain of the junction field effect transistor JFET is connected with the drain of the power switch M1, when the power switch M1 is turned off, the drain voltage of the power switch M1 is 100V-400V. Therefore, the junction field effect transistor JFET can charge the power supply capacitor Cvcc, and the charging process is controlled by the JFET control circuit. When the voltage of the power supply capacitor Cvcc is insufficient, the junction field effect transistor JFET is turned on; and when the voltage of the power supply capacitor Cvcc is sufficient, the junction field effect transistor JFET is turned off, and the supply of the JFET control circuit can continue several switching cycles of the power switch M1.
When the power switch M1 is switched from an off state to an on state, the drive circuit pulls a gate voltage of the power switch M1 from 0V to a power supply capacitor Cvcc voltage, the gate charge of the power gate M1 is provided by the power supply capacitor Cvcc; and when the power switch M1 is switched from the on stated to the off state, the drive circuit pulls the gate voltage of the power switch M1 down to 0V.
However, the above power supply capacitor Cvcc requires a relatively large capacity (typically calculated in terms of nanofarad nF or microfarad uF), therefore cannot be designed in an integrated circuit, and requires a separately packaged capacitor. Firstly, the power supply capacitor requires cost, and occupies space; secondly, the dedicated pins of the integrated circuit also occupies the system cost; and thirdly, the risk of system failure is increased.
Therefore, the existing power driver circuit has a problem that the capacity of the power supply capacitor is too large to be able to be designed on the integrated circuit, which results in a large occupied space and an increased cost.